In the field of optical communications, communication transceivers which transmit and receive an optical signal are facing a demand for size reduction. This is because reduction in size of a communication transceiver allows reducing a space occupied by the communication transceiver as well as suppressing production cost and power consumption of the communication transceiver. In order to meet the demand, vigorous research has been conducted on technology related to an optical integrated circuit in which an optical waveguide (substrate waveguide), a light-emitting device, an optical modulator, and the like are integrally mounted on a semiconductor substrate.
A problem of this optical integrated circuit is that optical loss tends to be increased in a case where the substrate waveguide constituting the optical integrated circuit is connected to an optical fiber.
This is because a refractive index of a semiconductor material constituting the substrate waveguide significantly differs from that of a dielectric material constituting the optical fiber. The significant difference in refractive index between the semiconductor material and the dielectric material results in a significant difference between a mode field of light traveling in the substrate waveguide and a mode field of light traveling in the optical fiber. In such a case where light and another light whose mode fields are significantly different from each other are directly coupled to each other, optical loss increases.
There is known a technique to suppress the increase in optical loss by approximating a mode field of one light to a mode field of the other light with use of, for example, a lensed fiber. The lensed fiber is an optical fiber whose end part is microfabricated to have a shape of, for example, a wedge or a cone. The lensed fiber has a mode field which is gradually converted toward the end part. This arrangement allows suppressing the optical loss which is caused in the case where the substrate waveguide and the optical fiber are connected to each other. In other words, the arrangement enables high coupling efficiency.